A wide variety of IMDs have been developed over the years or are proposed that provide cardiac rhythm management of disease states manifested by cardiac rhythm disorders and heart failure. Implantable pacemakers have been developed that monitor and restore heart rate and rhythm of patients that suffer bradycardia (too-slow or irregular heart rate), tachycardia (regular but excessive heart rate), and heart failure (the inability of the heart to maintain its workload of pumping blood to the body). Implantable cardioverter-defibrillators (ICDs) have been developed that deliver programmed cardioversion/defibrillation shocks to the atria, in response to detection of atrial fibrillation (rapid, uncontrolled heartbeats in the atria), or to the ventricles, in response to life-threatening, ventricular tachyarrhythmias. Typically, single and dual chamber bradycardia pacing systems are also incorporated into ICDs.
Cardiac IMDs have traditionally employed the ability to detect or sense electrical activity in the heart as the basis for determining and delivering appropriate therapy. For example, appropriately placed electrical sensors may sense the contractions of the atria and/or the ventricles as evidenced by P-waves and R-waves detected in atrial and ventricular electrogram (EGM) signals, respectively. The timing of detected atrial and ventricular contractions (sensed events) may be used by the IMD to monitor for and treat cardiac arrhythmias such as bradycardia, tachycardia, and fibrillation.
Among the earliest cardiac rhythm management IMDs were single-chamber, fixed-rate pacing systems comprising an implantable pulse generator (IPG) and a lead bearing one or more pace/sense electrodes adapted to be placed in contact with the heart chamber to be paced. These IMDs, commonly referred to as pacemakers, provided fixed-rate pacing to a single heart chamber when the heart rate fell below a programmable lower rate limit.
Another cardiac rhythm management IMD, the implantable cardioverter defibrillator (“ICD”), was developed for treating abnormally fast heart rhythms. The earliest ICDs delivered a defibrillation shock to the ventricles when heart rate, as determined by sensed ventricular contractions, and certain other criteria were met. It was proposed that blood pressure sensors or accelerometers be incorporated in ICDs so that the absence of mechanical heart function during fibrillation could also be detected to confirm the presence of fibrillation before a shock therapy was delivered.
Over the years, pacemakers and ICDs have evolved in complexity and capabilities. Increasingly complex signal processing algorithms have been developed to evaluate electrogram (EGM) signals and to thereby attempt to provide the most appropriate therapy to restore a normal heart rhythm and to avoid delivery of inappropriate therapy that may be painful and potentially harmful to the patient.
It has been recognized that other indicators of heart function, particularly indicators related to mechanical heart function, would be of great value in augmenting the algorithms that process atrial and ventricular EGM signals in order to resolve ambiguities that may arise. It is desirable, for example, to know whether a delivered pacing pulse has “captured” the heart, i.e., caused the heart chamber to contract. Similarly, it is desirable to rapidly determine whether a delivered cardioversion/defibrillation shock has effectively terminated a tachyarrhythmia and whether the heart has returned to a normal rhythm.
There are other situations where it would be useful to incorporate measurements or indications of mechanical heart function in pacing systems. For example, patients suffering from chronic heart failure or congestive heart failure (CHF) often manifest an elevation of left ventricular end-diastolic pressure. This may occur while left ventricular end-diastolic volume remains normal due to a decrease in left ventricular compliance. CHF due to chronic hypertension, ischemia, infarct or idiopathic cardiomyopathy may be associated with compromised systolic and diastolic function involving decreased atrial and ventricular muscle compliance. These conditions may be associated with chronic disease processes, or complications from cardiac surgery with or without specific disease processes. Most heart failure patients suffer from conditions which may include a general weakening of the contractile function of the cardiac muscle, attendant enlargement thereof, impaired myocardial relaxation, and depressed ventricular filling characteristics in the diastolic phase following contraction. Pulmonary edema, shortness of breath, and disruption in systemic blood pressure are symptoms associated with acute exacerbations of heart failure.
These disease processes often lead to insufficient cardiac output to sustain mild or moderate levels of exercise and proper function of other body organs; progressive worsening eventually results in cardiogenic shock, arrhythmias, electromechanical dissociation, and death. In order to monitor the progression of the disease and to assess efficacy of prescribed treatment, it is desirable to obtain accurate measures of the heart geometry, and the mechanical pumping capability of the heart, under a variety of metabolic conditions. These parameters have typically been measured through the use of external echocardiogram equipment in a clinical setting. However, the measurement procedure is time consuming and expensive to perform for even a resting patient, and cannot be practically performed while replicating a range of metabolic conditions. Typically, the echocardiography procedure is performed infrequently, and months or years may lapse between successive tests, resulting in a poor understanding of the progress of the disease or whether or not intervening therapies have been efficacious. Quite often, only anecdotal evidence from the patient is available to gauge the efficacy of the prescribed treatment.
It has been proposed to employ sensors that respond to mechanical activity of the heart to provide an indication of the strength, velocity or range of motion of one or more of the heart chambers or valves. It is desirable that such information complement information obtained from EGM signals to more confidently detect arrhythmias or trigger delivery of appropriate therapies. It is also desirable to derive indicators of intrinsic cardiac performance and response to delivered therapies that can be employed to confirm or adjust therapy delivery, or to indicate the state and progress of the underlying cardiac disease.
It has been proposed to employ permanently implantable sensors that provide a more direct measure of mechanical motion of muscle mass or particular structures of the heart, including the opening and closing of heart valves and the motion or deformation of the septal wall and the ventricular and atrial walls. Such sensors include intracardiac pressure sensors, accelerometers, impedance measurement electrode systems, and Doppler motion sensors.
As noted in U.S. Pat. No. 5,544,656, measurement of myocardial wall thickness, as well as end-systolic and end-diastolic dimensions, may be useful in evaluating the effects of changes in regional myocardial function and contractility, including evaluating myocardial oxygen supply and demand, in acute and chronic animal studies. A transit-time sonomicrometry system is disclosed in the background of the '656 patent that uses two piezoelectric crystals, one as a transmitter and the other as a receiver, and operates by measuring the time required for ultrasound to travel between the transmitting and receiving transducers. An advantage of this system is its ability to provide an absolute dimension signal output calibrated in units of distance.
The '656 patent also discloses a closed-loop, single-crystal, ultrasonic sonomicrometer capable of identifying the myocardial muscle/blood interface and continuously tracking this interface throughout the cardiac cycle using a piezoelectric transducer that operates in the manner of a Doppler echo sensor implanted at least partly in the myocardium and partly in the blood within a heart chamber.
Sonomicrometer systems that are installed epicardially about the heart to measure heart movement across a number of vectors are also disclosed in the article “Miniature Implantable Sonomicrometer System,” by Robert D. Lee et al., (Journal of Applied Physiology, Vol. 28, No. 1, January 1970, pp. 110–112), in EP0 467 695 A2, and in PCT publication WO 00/69490. The Lee article describes an implantable monitoring system attached to the epicardial electrodes. Invasive surgery is necessary to expose locations where sonomicrometer crystals may be surgically attached to the epicardium.
Some of the various chronically implanted sensors described above are intended to be incorporated into lead bodies that are typically introduced transvenously into the relatively low pressure right heart chamber or blood vessels accessible from the right atrium through the patient's venous system. The introduction of such sensors into left heart chambers through the arterial system introduces complications that may be difficult to manage both acutely and chronically. The surgical approach to the exterior of the heart is also not favored as it may complicate the surgery and recovery of the patient. However, measurement of left heart function remains desirable in a number of clinical cases including chronic heart failure.
Stadler et al. (U.S. Pat. No. 6,795,732) discloses a system and method for determining mechanical heart function and measuring mechanical heart performance of both left and right heart chambers without intruding into a left heart chamber or requiring invasive surgery to access the epicardium of the left heart chamber. The system disclosed by Stadler et al. may be incorporated in IMDs (for therapy delivery) and/or implantable monitoring devices employing dimension sensors, such as piezoelectric sonomicrometry crystals. U.S. Pat. No. 6,795,732 to Stadler et al. is assigned to the present assignee and is hereby incorporated by reference in its entirety.
The dimension sensors of Stadler et al. comprise at least a first sonomicrometer piezoelectric crystal mounted to a first lead body implanted into or in relation to one heart chamber, e.g., the right ventricle (RV), that operates as an ultrasound transmitter when a drive signal is applied to it or as an ultrasound receiver, and at least one second sonomicrometer crystal mounted to a second lead body implanted into or in relation to a second heart chamber, e.g., the left ventricle (LV), the left atrium (LA), or the right atrium (RA), that operates as an ultrasound receiver or as an ultrasound transmitter when a drive signal is applied to it, respectively. The ultrasound receiver converts impinging ultrasound energy transmitted from the ultrasound transmitter through blood and heart tissue into an electrical signal. The time delay between the generation of the transmitted ultrasound signal and the reception of the ultrasound wave varies as a function of distance between the ultrasound transmitter and receiver, which in turn varies with contraction and expansion of a heart chamber between the first and second sonomicrometer crystals. One or more additional sonomicrometer piezoelectric crystals can be mounted to additional lead bodies, such that the distances between the three or more sonomicrometer crystals can be determined. In each case, the sonomicrometer crystals are distributed about a heart chamber of interest such that the distance between the separated ultrasound transmitter and receiver crystal pairs changes with contraction and relaxation of the heart chamber.
The RV-LV distance between the RV and LV crystals of Stadler et al. is a measure of LV dimension. Changes in the LV dimension over the cardiac cycle are correlated with changes in LV volume as the LV fills during diastole and empties during systole. The LV-RA distance between the LV and RA crystals varies as a function of RA mechanical activity as the RA fills and empties in a pattern during normal sinus rhythm that markedly differs from the pattern exhibited during atrial fibrillation and other forms of ineffective atrial contraction. The RV-RA distance and RV-LA distance between the RV sonomicrometer crystal and the respective RA and LA sonomicrometer crystals varies as a function of a mixture of atrial and ventricular activity.
Stadler et al. discloses incorporating sonomicrometer piezoelectric crystals into cardiac leads, distributing sonomicrometer piezoelectric crystals about the heart chambers, and incorporating a control and measurement system in the operating system of an IMD that measures the distance between the sonomicrometer crystals as the heart expands and contracts over each heart cycle. First and second cardiac pacing leads or cardioversion/defibrillation leads bearing first and second sonomicrometer crystals, respectively, are implanted through the coronary sinus (CS) and into the great cardiac vein along the LV and in the RV apex, respectively. The lead conductors are coupled to emission, reception, and dimension measurement circuitry within an IMD IPG or monitor that drives one selected piezoelectric crystal as an emitter or generator and the other piezoelectric crystals as receivers, whereby the distances between the crystal pairs can be measured as a function of the measured transit time for the transmitted signal to be received by multiplying the time of travel by the speed of sound in the tissue.
Stadler et al. also discloses incorporating a sonomicrometer crystal into a pacing lead with two or more conductors such that the crystal is wired in parallel with two of the conductors in the lead. For example, a crystal could be wired in parallel with the ring and tip pace/sense electrodes of a pacing lead. The ultrasound crystal has very low impedance to signals near its resonance frequency (near 1 MHz), and very high impedance to lower frequency signals. Pacing pulses, which contain lower frequencies, would preferentially be delivered to the tissue via the tip and ring electrodes, whereas high frequency pulses to excite the ultrasound crystal would be preferentially delivered to the crystal. Additionally, the low pass filter of the pacing sense amplifier does not pass the very high frequency ultrasound signals emitted by the crystals. Thus, the sonomicrometer function does not interfere with normal pacing and sensing functions. As an alternative implementation, the pacing pulse could be delivered simultaneously to the tip and ring pace/sense electrodes, with the IPG case or can as an anode, thereby delivering an effective pacing pulse without any energy dissipation through the ultrasound crystal. As a second alternative, filtering circuitry could be incorporated into the lead to ensure delivery of pacing pulses to the pace/sense electrodes and ultrasound pulses to the crystals.
Pacing therapies delivered by implantable devices may, in some cases, cause asynchronous left ventricular contraction, particularly when pacing the right ventricular apex. In contrast, left ventricular, multi-site, or alternative site right ventricular pacing may lessen the asynchrony of left ventricular contraction (i.e., improve synchrony of left ventricular contractions). The ability of these pacing therapies to resynchronize ventricular contractions may depend on the precise pacing site locations, as well as on pacing parameters such as the programmed AV delay, VV delay interval, and possibly other programmable pacing parameters. However, no clinically accepted method currently exists to reliably evaluate and optimize cardiac performance using sensors in conjunction with an implantable device.